This invention relates to the production of cement and is particularly concerned with using spent solids produced during coal gasification and similar coal conversion operations to manufacture cement.
Catalytic and noncatalytic coal gasification processes and similar operations carried out at high temperatures generally result in the formation of chars. The chars normally include unconverted carbonaceous constituents of the coal or other feed material and various inorganic constituents generally referred to as ash. It is generally advisable to withdraw a portion of the char from the reaction zone during gasification and similar operations in order to eliminate the ash and prevent it from building up within the reaction zone or other vessels in the system. The amount of char removed will normally be quite large, over 20 weight percent of the feed in some instances, and therefore creates disposal problems.
In gasification and similar processes carried out in the presence of an alkali metal-containing catalyst, the resultant chars will contain alkali metal residues along with unconverted carbonaceous constituents and ash. In order to maintain catalyst cost at a reasonable level, it is essential that the alkali metal constituents be recovered from the char residue and reused in the process. There have been proposals for the recovery of alkali metal constituents by water leaching the char after it is withdrawn from the reaction zone and before it is sent to disposal. Such a procedure, however, only recovers the water-soluble alkali metal constituents. It has recently been found that increased amounts of alkali metal constituents can be effectively recovered from the char particles by treating the particles with calcium hydroxide in the presence of water at elevated temperatures and pressures. The calcium ions from the calcium hydroxide evidently react with the alkali metal aluminosilicates and other water-insoluble alkali metal compounds in the char particles thereby liberating alkali metal constituents which dissolve in the water to form an aqueous solution which is recycled to the reaction zone. The spent solids resulting from this treatment step must then be disposed of as waste material.
Regardless of whether a coal gasification process or similar coal conversion operation is carried out in the presence or absence of a catalyst, the spent solids withdrawn from the reaction zone must be disposed of in some manner. In the case of catalytic gasification, the spent solids will contain catalyst residues and may be further treated to recover catalyst constituents. However, spent solids containing carbonaceous material, ash and other constituents will remain after the catalyst recovery step and must be disposed of in a fashion similar to those produced in noncatalytic processes which do not employ catalyst recovery steps. Normally, the spent solids referred to above are transported from the process site to a dump area where they are used as landfill. Disposal in this manner is expensive since the quantity of the spent residue solids will be relatively large, and therefore transportation and other logistics will be complicated. Moreover, the composition of the residue solids may pose environmental problems that could add considerably to the cost of disposal. Thus, disposal of the waste solids is a significant factor in determining the overall cost of the conversion process and no benefit is gained by throwing away these materials, especially in view of the fact that the energy content of the carbon present is lost to the process.